The invention relates to a method and apparatus for obtaining a signal low in harmonics from a signal consisting of individual amplitude steps.
In telemetering, for instance, an analog measuring quantity is often quantized at the input of a transmission channel into a number of amplitude steps to obtain a transmission accuracy as high as possible. Related to each amplitude value is a definite pulse sequence through which a code is obtained. At the other end of the transmission channel, decoding is performed, so that again an analog measuring quantity is obtained which is put together from individual amplitude steps. This analog quantity has a relatively high harmonic content. The harmonics of the highest frequencies can be filtered out by means of a lowpass filter, which just passes the upper limit frequency of the signal; however, the harmonics of low-frequency processes are passed because the filter is constructed to the upper frequency limit. Also an analog measuring quantity filtered by means of a filter, which quantity is obtained from a measurement quantity consisting of individual amplitude steps, therefore still has a rather considerable harmonic content.
Further, in measurement technology, for example, in the measurement of noise, the measurement quantities are sometimes converted continuously into amplitude steps or digital data. These digital data are stored and converted back into analog form after a time delay. The quantity, which consequently consists of individual amplitude steps, is then also fed to filters, which filter out part of the harmonics and the sampling frequency. In this connection, reference may be had to Siemens Zeitschrift, vol. 45 (1971) No. 10, pages 634 and 635. A simultation, low in harmonics, of the measurement quantity at the output, as compared to the measurement quantity at the input, cannot be completely achieved in this manner.